When methyl methacrylate is graft polymerized onto a backbone of propylene polymer material, the product contains a certain amount of free monomer. When the resulting graft copolymer is subjected to melt processing, e.g., extrusion and/or molding, depolymerization occurs and additional monomer is formed. The free monomer in the graft copolymer can lead to surface irregularities known as "splaying" and can produce an odor in the finished product. The conventional method for limiting depolymerization is to copolymerize the methyl methacrylate with a monomer that acts as a chain stopper during the depolymerization reaction, e.g., methyl acrylate, although this method does not eliminate depolymerization entirely.
European patent application 726,289 discloses the addition of compounds containing a .dbd.N--O group, e.g., a 2,2,6,6-tetramethyl-1-piperidinyloxy group, to a polymer to inhibit undesirable reactions during heating. e.g., to inhibit depolymerization of polystyrene or poly(methyl methacrylate) during thermal processing, or to inhibit crosslinking of unsaturated polymers such as polybutadiene or butadiene-styrene copolymers during thermal grafting with compounds such as methacrylic acid.
U.S. Pat. No. 4,668,721 discloses the use of hydroxylamine derivatives to stabilize polyolefins against degradation or discoloration after high temperature extrusion, exposure to the combustion products of natural gas or to gamma irradiation, or upon storage for extended periods. U.S. Pat. No. 4,876,300 discloses the use of long chain N, N-dialkyl-hydroxylamines as process stabilizers for polyolefins that are processed at elevated temperatures.